Zion National Park:
A Tweak of History and a Tweak of Geology

by D. S. Grover

[Ed. Note: This article won third place in the adult
article competition in the California Federation
of Mineralogical Societies in 1996.]

Zion National Park, comprising 146,551 acres, is the site of a
beautiful canyon cut by the Virgin River into the vermillion and
white cliffs of southwestern Utah. It is entered between two
great blocks of stone, the West Temple and the Watchman.
Beyond the impressive gateway is the large canyon.

Against the east wall of the canyon stands the great White
Throne, a flat-topped dome. In vivid contrast to the Throne is
the dull red of Angels Landing directly opposite. The auto road
ends in a great parklike amphitheater -- the Temple of
Senowana. Beyond, a trail leads to the Narrows where the
canyon is 2,000 feet deep and, at one place, only 20 feet wide.

A Mormon scout, the first white man to see Zion, discovered
the canyon in 1858. He reported it to the church, and the
leaders were so impressed by the majesty and grandeur of the
rocks, colors, and peaks that they named all of the features of
the canyon as they related to the religious connotations of the
Mormon Church.

The earliest rocks of Zion were born when mud, silt, and sand
settled in ancient swamps and along streams. Later, massive
dunes, formed from windblown sand, covered the muddy
underlayers. These raw materials eventually formed the rocks
we see today -- the slope-forming shale and the cliff-forming
sandstone. The land mass, as it accumulated layers of sediment
thousands of feet thick, began to sink. The sinking occurred at
about the same rate as the materials piled up, so the elevation
remained nearly constant. Eventually, the land sank below sea
level and was flooded by a shallow inland sea. Water seeped
down between the grains of sand and other particles, carrying
minerals with it. Together with the tremendous force of
compression, these minerals cemented the sand grains together,
turning them into stone.

Much later, forces deep in the earth caused the land mass to
rise. This slow, steady uplift occurred over a broad area --
namely the entire Colorado Plateau, of which Zion is a small
part. Portions of the plateau were raised over 10,000 feet. The
uplift is still occurring today. As the uplift occurred, rivers and
streams which once meandered slowly across flat plains began
to pick up speed. Tumbling rapidly off the edge of the plateau,
they began to carve into the rock layers. The Virgin River
carries sediment continuously, and hauls masses of mud, sand,
and boulders during floods. Over time, the north fork of the
Virgin River has carved this great chasm called Zion Canyon.

Zion Canyon is famous for its majestic 2,000-foot-high vertical
walls of Navajo Sandstone. This formation resulted from the
massive sand dunes present in the area during the Jurassic
period, about 170 million years ago. During this period,
dinosaurs flourished elsewhere. The desert climate in this area
supported only sparse plant and animal life; thus, few fossils are
found in the Navajo layer.

East of the Zion-Mt. Carmel Tunnel lies the white upper layer
of the Navajo Sandstone. Without iron oxide, the rock is white
rather than red, is weaker due to fewer cementing materials,
and erodes easily into domes and buttes. Alternating horizontal
lines in the rock, called "cross-bedding," reveal the exposed
layers of ancient sand dunes. Often called "slickrock," this weak
rock often has a layer of loose sand on the surface. Watch your
step.

Although the rocks are fairly old ("middle age" by geologic
standards) the canyon itself is "quite recent." Geologists believe
the uplift and canyon carving began no earlier than 10 to 12
million years ago. By this time dinosaurs were long gone,
mammals were well established, and the Rocky Mountains had
already formed.

The cliffs of Zion are stained and streaked with a variety of
bright hues. Iron oxide causes shades of red, orange, green, and
brown. Lichens and moss growing on the rock appear white,
gray-green, black, and bright orange. Dull black streaking may
occur where organic matter has been deposited by water runoff.
Shiny black surfaces, called desert varnish, contain iron and/or
manganese.

Autumn color displays begin in September in the high country
and peak in Zion usually in late October. Winters are fairly
mild. Winter storms often bring rain or light snow to Zion, but
heavier snow to the higher elevations. Clear days may become
quite warm, reaching 60 degrees or more. Nights are in the
20 - 40 degree range. Summer highs in July and August are
generally 90 to 100 degrees.

References

The preceding article was originally published in the February 1996
issue of Lithosphere, the official bulletin of the
Fallbrook [California] Gem and Mineral Society, Inc; Richard Busch
(Editor).

Permission to reproduce and distribute this material, in
whole or in part, for non-commercial purposes, is hereby granted
provided the sense or meaning of the material is not changed and
the author's notice of copyright is retained.